I'm not sure, you'll have to poke around the manuals and diagrams online to figure that out. Is the original one intact, just attached to the end of the broken crank? You might be able to salvage that.
Look like you'll need to replace the whole crank, fortunately it's pretty affordable - $18 from Traxxas themselves, part #4021
Edit: assuming you need the long-crank version. Short-crank is #4019